The present invention relates to a system for controlling an automatic transmission for a vehicle having an infinitely variable belt-drive transmission associated with an electromagnetic powder clutch.
In Japanese patent laid-open specification No. 55-63051, an infinitely variable belt-drive transmission associated with a centrifugal clutch is disclosed as an automatic transmission system for a vehicle. Further, Japanese patent laid-open specification No. 55-65755 and U.S. Pat. No. 4,152,947 disclose an infinitely variable belt-drive transmission, the transmission ratio of which is automatically varied in accordance with driving conditions of the vehicle.
FIG. 3 shows relationships between vehicle speed and engine speed of a vehicle provided with such an automatic transmission. As seen from the heavy, arrowed curve in FIG. 3, the centrifugal clutch begins to engage when the engine speed reaches a speed V.sub.1 which is a little higher than an idle speed Vi, and the clutch engages entirely at a point P.sub.1 when the engine speed reaches to a predetermined speed V.sub.2. Then, the vehicle is driven at a maximum transmission ratio. Therefore, engine speed and vehicle speed increase as the accelerator pedal of the vehicle is depressed. Since the transmission ratio is kept at the maximum transmission ratio of the infinitely variable transmission, the engine speed and vehicle speed increase along a straight line 1.sub.1 as shown by a line A. When the relationship between the depression degree of the accelerator pedal and the engine speed reaches a proper condition, the transmission ratio starts to change to decrease the ratio. Accordingly, the line A representing the engine speed and vehicle speed on the line 1.sub.1 changes a horizontal line B. When the transmission ratio reaches to a minimum ratio, the engine speed and vehicle speed change along a line 1.sub.2. Upon decelerating while the transmission ratio is minimum, both values of the engine speed and the vehicle speed descend along the line 1.sub.2 as shown by a line C. Further, when the engine speed decreases to a point P.sub.2, that corresponds to the engine speed V.sub.2 which is a small value as long as the centrifugal clutch can be kept in engagement, the transmission ratio continuously increases, whereby the vehicle speed decreases as shown by a line D (this region is hereinafter called "shiftdown"). When the vehicle speed reaches the line 1.sub.1 which represents the maximum transmission ratio, the engine speed also decreases along the line 1.sub.1 as line E. When the engine speed decreases to the speed V.sub.1, the clutch is disengaged at a point P.sub.3.
Thus the engine speed P.sub.2 during the shiftdown excursion D is determined at a value approximate to the speed V.sub.2 so as to keep the centrifugal clutch in the engaging state, which depends on the characteristics of the centrifugal clutch. If the engine speed P.sub.2 is shiftdown is set to a lower value than the speed V.sub.2, the clutch slips when accelerating the vehicle during the shiftdown excursion, which results in trouble, such as overheating or breakdown of the clutch.
In order to prevent such slipping of the clutch, the engine speed P.sub.2 during shiftdown must be set to a higher value. Thus, fuel consumption of the engine inevitably increases. Further, the transmission ratio reaches a maximum ratio on the line 1.sub.1 at a considerably high engine speed and there is a drastic engine breaking effect on the vehicle, which will cause a decrease of drivability of the vehicle.